Casting is a fabrication technique which is presently in widespread use in conjunction with a variety of materials. Casting of metals allows for the economical fabrication of variously shaped metallic items without the need for machining, stamping or other such metal working processes. In general, casting involves the introduction of molten material into a mold, cooling of the material and removal of the finished item from the mold.
In many instances the shape of the finished item is such that it is not readily removable from the mold. For example, the item may include undercut regions or other complex shapes precluding ready demolding. In other instances, it is desirable to fabricate a hollow article and in yet other instances it may be desirable to mold screw threads or other such features into a casting. In order to mold these various shapes, casting cores are generally employed. These cores are formed from a heat resistant material and are used to constrain the molten metal into a particular shape. For example, in the casting of a hollow article, a core will be placed in a mold so as to substantially fill the mold, leaving only a relatively thin "shell" to be filled by the subsequently introduced molten metal. In those instances where it is desirable to cast screw threads into a metal body, a screw shaped core is incorporated in the mold. After the cast metal has solidified, the core is removed leaving behind impressions of the screw threading.
After the casting process is complete it is necessary to remove the cores and toward that end they are frequently fabricated from a frangible material such as a sand/organic resin composite which may be readily broken out of the casting. While such sand cores are relatively cheap they are not capable of providing a high quality surface finish or maintaining precise dimensional tolerances and hence are not suitable for casting of precise shapes such as screw threads and the like.
Another approach to the problem of providing readily removable casting cores involves the use of ceramic coated synthetic polymeric foam bodies. In the casting procedure, the organic matter comprising the foam burns away while the ceramic facing provides for a smooth metal finish. This process is frequently called the "lost foam" process and provides higher quality casting than does a sand core process. Such cores are relatively difficult to fabricate, fragile, expensive, and not well suited for the casting of screw threads. For this reason alternatives to the lost foam process capable of providing high quality castings have been sought.
Many salts are capable of being melted and cast into a variety of shapes having a relatively smooth surface finish capable of withstanding high temperatures encountered in a metal casting process, hence efforts have been undertaken to use such materials as casting cores. By use of an appropriately water soluble salt, such cores may be fabricated to be readily removed by a water wash process. U.S. Pat. No. 3,356,129 discloses the use of water soluble salt cores in a casting the process. In other instances, thermal energy may be utilized to melt a core out of a casting as will be explained in greater detail hereinbelow.
Problems can occur in the use of salt cores because of the physical properties of most readily available salt materials. Generally molten salt shrinks upon cooling making the maintenance of precise tolerances difficult. Additionally, such cast salt materials are relatively fragile and frequently manifest poor surface quality due to cracking, spalling and other damage thereto. In yet other instances cast salt materials are hygroscopic and tend to absorb atmospheric moisture which degrades the surface finish thereof; and in yet other instances the materials used for the cores react with the casting metal.
Control of thermally induced dimensional changes is very important in processes involving casting cores. As a first requirement, the core must not undergo extreme dimensional changes during its fabrication, since such changes can cause spalling, cracking or other surface damage to the core, as well as result in the loss of dimensional tolerances. Also, the core must be thermally stable during the casting process, that is to say it must not be damaged by thermal shock and it should have a thermal coefficient of expansion similar to the metal being cast. Heretofore employed salt cores tended to change dimensions significantly as they cooled from the melt; however, it has been found that cores prepared according to the present invention have high thermal stability. Another parameter to be considered in the use of casting cores is their thermal conductivity. In general, it is desired that casting cores have relatively low thermal conductivity, so as to prevent undue heating and possible melting of the core during the casting process. Low thermal conductivity eliminates distortion and edge melting of the cores, particularly where they are used at temperatures near their melting point.
Obviously, it would be desirable to increase the strength, dimensional stability and surface quality of salt cores so as to provide for the improved casting of materials particularly metals. U.S. Pat. No. 1,523,519 teaches the fabrication of for the vulcanization of rubber articles, which cores are fabricated from sodium and potassium nitrate combinations and which may be filled with an inert material such as mineral flour. As taught therein, the us of mineral flour lowers the cost of the casting mixture and also serves to weaken and embrittle the cast core so that it may be more readily broken up and removed from the finished article. There is no teaching whatsoever therein of the use of any reinforcing material in a cast salt core to increase the strength and/or surface finish thereof. U.S. Pat. No. 3,692,551 shows the manufacture of cores suited for the relatively low temperature casting of plastic resins. The cores of the '551 patent are manufactured from a molten salt which may include sand or other such inert filler material therein. There is no teaching in the '551 patent of any strength increasing or dimensional stabilizing function for the filler or the use of various combinations of filler particle size to obtain a high quality core.
U.S. Pat. No. 3,459,253 describes a process for casting cooling passages into pistons, which process relies upon the use of water-soluble, casting cores. As disclosed therein, the cores are preferably fabricated from a sulfate/carbonate salt mix and may include a unitary wire or glass fiber reinforcing matrix as well as optional fillers. The '253 patent does not teach or suggest the use of reinforcing and/or shrinkage reducing fillers comprised of two different size of particulates, nor does it discuss the role of a filler material in controlling dimensional stability.
It will be appreciated that there is a need for water soluble, water disintegratable or readily meltable cores for use in a metal casting process, particularly an aluminum casting process, which cores are cheap and easy to manufacture, provide a good surface finish resistant to atmospheric moisture and a high degree of strength and dimensional stability. The present invention provides for the manufacture of high quality casting cores from molten salt material reinforced with a substantially inert material having a particular size distribution. The cores of the present invention may be fabricated to have a coefficient of thermal expansion similar to that of aluminum. They are readily disintegrated in water or remelted to facilitate their ready removal and present a high quality, ceramic-like finish. These and other advantages of the present invention will be readily apparent from the discussion, examples and claims which follow.